1. Field of the Invention
The present invention relates to a particulate detection system which measures the amount of particulates such as soot contained in a gas under measurement.
2. Description of the Related Art
Conventionally, a particulate detection system has been known which measures the amount of particulates (e.g., soot) contained in a gas under measurement (for example, exhaust gas discharged from an internal combustion engine or the like) (see, for example, Patent Document 1).
Such a particulate detection system includes, for example, a particulate sensor, a high voltage generation section, a primary current control section, and a sensor signal processing section (Patent Document 1).
The particulate sensor is exposed to the gas under measurement, and generates a sensor signal corresponding to the result of detection of particulates. The high voltage generation section includes a transformer having a primary winding and a secondary winding. The high voltage generation section supplies to the particulate sensor a high voltage generated from the secondary winding as a result of switching the supply of primary current to the primary winding. The primary current control section controls the primary current flowing through the primary winding. The sensor signal processing section processes the sensor signal generated by the particulate sensor.
The above-described particulate detection system includes an isolation transformer to suppress the influence, on the sensor signal processing section, of electromagnetic noise generated in the secondary winding. Notably, the isolation transformer is configured such that its primary-side and secondary-side cores are separated from each other with a small gap formed therebetween so as to isolate the primary-side and secondary-side cores from each other. Further, a common magnetic flux passes through the primary-side and secondary-side cores, whereby a transforming action is achieved.
The particulate detection system also includes an inner metal case and an outer metal case for shielding electromagnetic noise. The inner metal case surrounds the secondary winding, the secondary-side core, and the high voltage generation section. The outer metal case surrounds the inner metal case, the primary winding, the primary-side core, the primary current control section, and the sensor signal processing section.
The particulate detection system having such a configuration can suppress the influence, on the sensor signal processing section, of electromagnetic noise generated in the secondary winding. This is because the inner metal case can shield the electromagnetic noise generated in the secondary winding. As a result, it is possible to suppress a decrease in particulate detection accuracy due to the electromagnetic noise generated in the secondary winding.
[Patent Document 1] Japanese Patent Application Laid-Open (kokai) No. 2012-220423
3. Problems to be Solved by the Invention
In the above-described particulate detection system, a member for blocking electromagnetic noise is provided between the primary winding and the sensor signal processing section. Therefore, when electromagnetic noise is generated in the primary winding, the generated electromagnetic noise may influence the sensor signal processing section.
Notably, the primary winding generates a lower voltage as compared with the secondary winding, and the electromagnetic noise generated in the primary winding is of smaller magnitude than that generated in the secondary winding. Therefore, the influence of the electromagnetic noise generated in the primary winding on the sensor signal processing section is smaller than the influence of the electromagnetic noise generated in the secondary winding.
However, in the case where the sensor signal has a very small current value, the electromagnetic noise generated in the primary winding may affect the signal processing in the sensor signal processing section and lower the particulate detection accuracy.